Cisco SPA1001 Administration Guide - Page 62
Secure Call Implementation - setup
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Secure Call Implementation Chapter 3 Configuring Linksys ATAs Secure Call Implementation This section describes secure call implementation with a Linksys ATA. It includes the following topics: • Enabling Secure Calls, page 3-10 • Secure Call Details, page 3-10 • Using a Mini-Certificate, page 3-11 • Generating a Mini-Certificate, page 3-11 Enabling Secure Calls A secure call is established in two stages. The first stage is no different from normal call setup. The second stage starts after the call is established in the normal way with both sides ready to stream RTP packets. I In the second stage, the two parties exchange information to determine if the current call can switch over to the secure mode. The information is transported by base64 encoding embedded in the message body of SIP INFO requests, and responses using a proprietary format. If the second stage is successful, the Linksys ATA plays a special Secure Call Indication Tone for a short time to indicate to both parties that the call is secured and that RTP traffic in both directions is being encrypted. If the user has a phone that supports call waiting caller ID (CIDCW) and that service is enabled, the CID will be updated with the information extracted from the Mini-Certificate received from the remote party. The Name field of the CID will be prepended with a '$' symbol. Both parties can verify the name and number to ensure the identity of the remote party. The signing agent is implicit and must be the same for all Linksys ATAs that communicate securely with each other. The public key of the signing agent is pre-configured into the Linksys ATA by the administrator and is used by the Linksys ATA to verify the Mini-Certificate of its peer. The Mini-Certificate is valid if it has not expired, and it has a valid signature. The Linksys ATA can be configured so that, by default, all outbound calls are either secure or not secure. If secure by default, the user has the option to disable security when making a call by dialing *19 before dialing the target number. If not secure by default, the user can make a secure outbound call by dialing *18 before dialing the target number. However, the user cannot force inbound calls to be secure or not secure; that depends on whether the caller has security enabled or not. The Linksys ATA will not switch to secure mode if the CID of the called party from its Mini-Certificate does not agree with the user-id used in making the outbound call. The Linksys ATA performs this check after receiving the Mini-Certificate of the called party Secure Call Details Looking at the second stage of setting up a secure call in greater detail, this stage can be further divided into two steps. 1. The caller sends a "Caller Hello" message (base64 encoded and embedded in the message body of a SIP INFO request) to the called party with the following information: • Message ID (4B) • Version and flags (4B) • SSRC of the encrypted stream (4B) 3-10 Linksys ATA Administrator Guide Document Version 3.1